In a chemical reaction, a chemical change takes place. The reactants lose their properties, and the products formed usually have properties different from the reactants. This happens in a combustion reaction too. For example, on igniting, grey iron or steel wool burns in air with a bright light and a blue-black brittle substance remains as the residue, which is heavier than steel wool. In the combustion reaction, iron reacts with oxygen in the air. The new substance formed in this oxidation reaction is iron oxide. The term ‘oxidation’ is derived from the Latin word oxygenium, meaning oxygen. The more easily the electrons move from the outermost shells, the more reactive the substance is. Noble gases such as argon and neon are not reactive. For this reason, we breathe in the same argon atoms today which dinosaurs breathed in.
The Earth has a huge magnetic field and all magnetic objects, such as the compass needle, get oriented to it. One end of the freely rotating compass needle marked in colour aligns itself with the north pole of the Earth. This helped the early explorers to find their ways in oceans. Near the poles they often saw an impressive phenomenon in the sky known as aurora. Moreover, a lot of things would not have existed today without strong magnets called electromagnets that can be switched on and off.
A bottle opener is a simple lever. It intensifies the force with which we remove the crown cap of a bottle. Levers are mostly rigid, rod-shaped objects that have a fulcrum at one place. The longer the arm of the lever, the lesser the force required to do the work. The law of physics ‘Work = Force x Distance’ is applicable here too. The bottle opener is a one-sided lever. The longer the arm of the bottle opener, the easier it is to open it. Other examples of levers are pliers, scissors, crowbars, wrenches, paper punchers, and door handles.
The body’s centre of gravity normally lies near the belly button. When we bend forward and lift a heavy box, the centre of gravity of the ‘body plus the box’ lies far away from our body. This puts a lot of pressure on the backbone, and the back muscles resist this. In contrast, when we crouch down and get up with the box, the centre of gravity remains near the body. This puts less pressure on the backbone and helps keep our backs healthy.
Heavy furniture exerts a high pressure - force per unit area - downwards. If both the contact surfaces of the furniture and the surface of the floor are very rough, the surfaces adhere to each other. To move the furniture we have to overcome this force known as static friction. It is smaller if the surfaces are smoother.
A pulley is a device used to lift heavy objects. It is made with a cable and a wheel of different sizes. Here again, the law ‘Work = Force x Distance’ is applicable as in the case of serpentines. Here ‘distance’ means the length of the cable. The cable is wound around the wheels. The more the number of wheels, the smaller the force to be applied. We can find such pulleys on sailing ships for hoisting the sail. They are also used at construction sites to lift beams or other construction material. Earlier, pulleys were used to put heavy ship boxes in storage spaces high above the ground.
Any work can be done in many ways. What matters is how it is done. We can climb the mountain in two ways. If we take the shortest route, we need to apply a lot of force. If we gradually ascend towards the top using the serpentine roads, it takes long, but saves energy. It is based on the law of physics: Work = Force x Distance.
Spacesuits allow astronauts to survive and work when they go for a ‘walk’ in space. There is a supply of oxygen in spacesuits. They are designed to insulate astronauts from severe temperature changes experienced in space and to protect them against radiations from space. Spacesuits contain hollow spaces, which are filled with air when there is very less pressure on the body. This is important because the outer air pressure on the body is not present in space. In absence of these spaces blood may get collected in the lower half of the body. As a result, due to lack of blood supply to the brain one may become unconscious.
The chairs of flying swings do not fly away because of the long chains holding them. Similarly, satellites remain in their orbits because of the gravitational attraction of the Earth. At the same time they tend to ‘fly’ away from the Earth owing to its inertia. Finally, they are shot off from the Earth with a large force. If this initial thrust is measured properly while launching the rocket, the satellite continues to move in an orbit around the Earth at the calculated speed.
All objects have a mass. Because of the force of attraction on the Earth, an object has a weight. This is the force with which the body - that is, the mass of the body - is attracted. The force of gravity on the Moon is just one-sixth of that on the Earth. A person who weighs 6o kg on the Earth weighs only 10 kg on the Moon. The density of an object is the ‘mass per unit volume, i.e., space occupied by 1 kg of a substance. Because of the difference in their densities 1 kg of iron fits in a small cube, and 1 kg of feathers fills a cupboard completely.
Objects fall because the mass of the Earth attracts them. The force that pulls us towards the centre of the Earth is known as gravitation. It keeps us on the ground and the satellites in their orbits. This force of attraction is much less on the Moon than on the Earth - only one-sixth. For this reason, the astronauts bounce when they walk on the moon because they experience weightlessness. In the state of weightlessness we do not feel any force of gravity and do not feel any weight. However, we feel the forces acting on us on the Earth, when we fall from a height or when we are thrown forward when brakes are applied.
Gas is one of the three states of matter. In a gas, the forces that bind its elements together are not as strong as those in liquids or solids. These forces are not to be confused with the forces of attraction within the molecules. There are transitions between the states of matter. For instance, wax is solid at room temperature, melts when a candle is lit, and evaporates at a very high temperature. The fourth state of matter, rarely found on the Earth, is plasma. In this state the matter is ‘ionized’ and conducts electricity. 
An element is a ‘pure substance’ consisting of atoms of the same kind. Elements cannot be broken down into simpler substances, but they can combine with other elements to make new substances. Each element has a unique symbol, for example, ‘O’ means oxygen, ‘Na’ means sodium (Latin: natrium), and ‘CI’ means chlorine. The number of protons in the atomic nucleus determines the element. This number is called the atomic number of the element.
The term ‘atom’ comes from the Greek word atomos, meaning ‘indivisible’. Earlier, it was considered that atoms were the smallest unit of a substance and could not be split further. Today, we know that atoms can be further split into smaller particles. An atom is made up of the atomic nucleus, which consists of positively charged protons and an equal number of neutrons, and atomic shells. These are occupied by negatively charged electrons, which move at high speeds around the atomic nucleus - like planets around the sun.
